TEMPERATURE EFFECT ON METHYL-BROMIDE VOLATILIZATION IN SOIL FUMIGATION

Recent interest in characterizing methyl bromide emission has focused on field and laboratory measurements, which are expensive to conduct a nd very time consuming. Predicting methyl bromide volatilization with numerical or analytical models has been limited to idealized situation s without considering environmental conditions such as diurnal tempera ture change. It has been found that temperature can strongly affect me thyl bromide volatilization under field conditions. To quantitatively characterize temperature effect, we adopted a two-dimensional numerica l model that can solve simultaneous equations of water, heat, and solu te transport (including both liquid and vapor phases). Functional rela tionships were established between temperature and methyl bromide liqu id-gas phase partition coefficient or the Henry's constant, diffusion coefficient in soil air space, and the permeability of polyethylene ta rp. To test the model, soil properties and boundary conditions from Ya tes et al. (1996 a,b,c) were used. The model prediction was completely independent of the field measurement. The model simulation by conside ring diurnal variations of soil temperature predicted the cumulative e mission that agreed well with the measured flux density. Prediction wi thout considering temperature missed the diurnal nature in emission fl ux density. Comparable results were also obtained for methyl bromide c oncentration in the soil profile. The key advantage of this model is i ts ability of describing diurnal variations in methyl bromide emission flux. Based on the temperature effect on temporal variations of methy l bromide emission, we believe that small sampling intervals are neede d to determine the dynamic nature of methyl bromide emission under fie ld conditions, especially during the first 24 h after application.